Biomarkers HRV tracker accuracy and testing positions standing vs sitting?

 I'm using 3 different HRV  monitors and get different results?????  

I use Garmin 1040, Elite HRV with Garmin chest strap, and Fitbit Versa. I have been tracking my HRV daily for the past 5 weeks and sometimes Garmin is way off form the other two and Garmin 1040 has only given me one Green Day in five weeks. Elite HRV and Fitbit have given me mulitple days of Green durng the past 5 weeks.
My monitoring protocol is as follows:
I use a Red Amber Green system.
I take it my HRV immediately upon waking up with both Garmin 1040 and Elite HRV. Fitbittracks duing a 3hr sleep block. I track iwth both Elite and Garmin simultaneously. Garmin 1040 tracks for 3 minutes in the standing position. Elite tracks for two minutes and offers a variety of positions to choose. I use standing for both.

My questions are:
What is the best HRV tracker on the market?
Does anyone know why Garmin 1040 readings are so diffferent from the other two trackers Fitbit and Elite HRV??
What is the optimal position to monitor HRV standing, sitting, or lying down?